This application claims benefit of Japanese Patent Application No. 2000-302444 filed on Oct. 2, 2000, the contents of which are incorporated by the reference.
The present invention relates to image coding methods and, more particularly, to methods of suppressing the deterioration of image data transmitted on low quality communication lines.
In a prior art image sequence coding method, in which intra-frame coding (or intra-frame coding) and inter-frame predictive coding are selectively used for each block as unit of coding, for suppressing image quality deterioration due to errors occurring in the transmission of coded image sequence, it is in practice to make forced block intra-frame coding aside from coding mode selection on the basis of the coding efficiency. Inter-frame predictive coding of deteriorated quality images results in extreme deterioration of the subjective image quality due to propagation of the deterioration over a plurality of frames. By forcedly intra-frame coding images, on the other hand, the propagation of the deterioration can be suppressed to improve the resistance against the image quality deterioration. The degree of deterioration due to occurrence of errors in individual areas of image, depends on the pixel value distribution. Thus, the resistance against errors can be effectively improved without great sacrifice in the coding efficiency by preferentially making forced intra-frame coding pronouncedly deteriorated image quality blocks.
FIG. 8 is a flow chart showing the processing flow of an image sequence coding method adopting the above prior art. This prior art image sequence coding method will now be described with reference to FIG. 8.
When coding input image for each frame, the image quality deterioration degree representing the degree of image quality deterioration in the case of assuming missing of coded data due to an error, is first computed for each block as unit of coding (step S20). Then, the degree of preference for the forced intra-frame coding is computed on the basis of the image quality deterioration degree (step S21). Then, blocks to be intra-frame coded are determined on the basis of the forced intra-frame coding preference degree. Aside from this, a coding mode taking the coding efficiency into considerations is selected (step S22). The blocks in the frame are then fully subjected to the intra-frame coding or the inter-frame predictive coding in dependence on the coding mode selected in the above way (step S23).
FIG. 9 is a block diagram showing an image sequence coding system show in Japanese Patent Laid-Open No. 2000-165881. The above prior art image sequence coding method will further be described with reference to FIG. 9.
When input image 101 is inputted, as a pre-processing, an inter-frame motion sensor 3 detects motions from a reference image stored in a frame memory 2, and feeds corresponding motion vectors to a motion compensation inter-frame predictor 20. At the same time, the sensor 3 feeds the sum of absolute differences between reference image and input image of the blocks obtained at the time of the motion detection to a deterioration error computer 4.
The deteriorating error computer 4 computes a deterioration error between deteriorated image and input image when it is assumed the occurrence of a transmission error in coded data.
FIG. 10 is a view for describing error at the time of deterioration. In the absence of any error, a decoder uses pixel values of blocks in the reference image at a motion compensation position thereof. On the other hand, when missing of data occurs due to error occurrence, the missing pixel values are extrapolated by directly copying the pixel values of the like position blocks in the reference image. The degree of image quality deterioration at this time, is expressed as the difference between the like position pixel values in the reference image and present frame pixel values.
Referring again to FIG. 9, a counter 21 and a data missing probability estimator 22 estimate the probability of occurrence of data missing due to an error for each block from the preceding frame coded data.
FIG. 11 is a view for describing error occurrence position and data missing probability. The coded data as shown is in the form, in which sync. codes are inserted between coded data each comprising a plurality of blocks. When the decoder detects an error, it discards data from that position up to the next sync. signal, and decodes data afresh from that position. For this reason, while errors occur randomly at the transmission time, the coded data is subject to discarding with the higher probability the greater the distance from the preceding sync. signal.
Returning again to FIG. 9, a forced intra-frame coding preference degree computer 30 computes the preference degree of making the forced intra-frame coding by threshold checking the deterioration error of the present frame and the data missing probability of the present frame. A mode controller 32 determines blocks to be subject to forced intra-frame coding from a forced intra-frame coding map 31 with forced intra-frame coding preference degree data stored therein. A switch 13 switchingly selects either the intra-frame coding or the inter-frame predictive coding as block coding mode. In the coding mode thus selected, the input image or predictive error signal is subjected to conversion coding and quantization and then converted to coded data for each block. In this method, with the blocks which are the greater in the deterioration error and the data missing probability, the degree of deterioration in the event of error occurrence is the greater. Such blocks are thus preferentially subjected to the intra-frame coding.
In the above prior art method, the forced intra-frame coding is made independently of the intra-frame coding/inter-frame predictive coding taking the coding efficiency into considerations. Therefore, depending on the setting of the threshold as reference of the forced intra-frame coding selection, the intra-frame coding may be selected for many blocks to result in undesired coding efficiency reduction.
In addition, in the prior art method the actual image quality deterioration in the event of the transmission error occurrence is not sufficiently reflected. Particularly, the effect of propagation of the deterioration generated in past frames by the inter-frame predictive coding is not taken into considerations.